High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319, and in Pengilly, U.S. Pat. No. 3,047,539, incorporated herein by reference. These patents disclose that the polyesters are particularly advantageous as film and fiber formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection moldable compositions. Further, poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Work pieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
Furthermore, in particular, poly(1,4-butylene terephthalate) is much simpler to use in injection molding techniques than poly(ethylene terephthalate). For example, it is possible to injection mold poly(1,4-butylene terephthalate) at low mold temperatures of from about 30.degree. C. to 60.degree. C. to produce highly crystalline, dimensionally stable moldings in short cycle times. On account of the high rate of crystallization, even at low mold temperatures, no difficulty is encountered in removing the moldings from the molds. Additionally, the dimensional stability of poly(1,4butylene terephthalate) injection moldings is very good even at temperatures near or well above the glass temperature of poly(1,4-butylene terephthalate).
Simultaneously with the development of injection molding grades of polyester resins, fiber glass reinforced compositions were also provided. See for example, Furukawa et al, U.S. Pat. No. 3,368,995 and Zimmerman, U.S. Pat. No. 3,814,725, incorporated herein by reference. These injection moldable compositions provided all of the advantages of the unfilled polyesters and, also because of the glass reinforcement, the molded articles had higher rigidity, yield strength, modulus and impact strength.
Furthermore, stable polyblends of poly(1,4-butylene terephthalate) and poly(ethylene terephthalate) can be molded into useful unreinforced and reinforced articles. See Fox and Wambach, U.S. Pat. No. 3,953,394, incorporated herein by reference. Additionally, copolyesters and block copolyesters containing units derived primarily from poly(1,4-butylene terephthalate) and from aromatic/aliphatic or aliphatic polyesters are also known. See e.g., U.S. patent application Ser. No. 752,325, filed Dec. 20, 1976, now abandoned, incorporated herein by reference. Such copolyesters and block copolyesters are useful per se as molding resins and also in intimate combination with poly(1,4-butylene terephthalate) and/or poly(ethylene terephthalate). These compositions are said to show enhanced impact strength.
It is also known to add polyolefins, especially high pressure low density polyethylene and high pressure high density polyethylene, to thermoplastic polyesters to enhance or provide certain properties. For example, Rein et al, U.S. Pat. No. 3,405,198, disclose the use of polyethylene in poly(ethylene terephthalate) as an impact modifier. Holub et al, U.S. Pat. No. 4,122,061, disclose polyester compositions which comprise a poly(1,4-butylene terephthalate) resin, a poly(ethylene terephthalate) resin, a fibrous glass reinforcement, alone or in combination with a mineral filler and, as an impact modifier therefor, a polyolefin or olefin based copolymer resin including polyethylene and propylene-ethylene copolymer. Cohen et al, U.S. Pat. No. 4,185,047, disclose the use of high pressure low density polyethylene in thermoplastic polyester compositions, particularly poly(ethylene terephthalate) and poly(1,4-butylene terephthalate) for improved mold releasability. All of the aforementioned patents are incorporated herein by reference.
More recently, it has been discovered that linear low density polyethylene when added to an aromatic polycarbonate resin results in molding compositions having improved weld line strength and heat stability while retaining their good impact strength at increased part thickness as compared to control samples of either standard polycarbonate or a commercial polycarbonate blend with polyethylene. See for example Research Disclosure No. 20819 p. 309 August 1981. Further, copending patent applications Ser. Nos. 444,228 and 444,229, filed Nov. 24. 1982 and Ser. No. 521,199, filed Aug. 8, 1983 disclose compositions comprising linear low density polyethylene and thermoplastic polyesters or polyester blends and impact modified polyester-polycarbonate blends, respectively, having improved compatibility, weld line strength, flow properties and mold releasability, including reduced plate out. The foregoing references are incorporated herein.
It has now been discovered that thermoplastic polyester compositions show unexpected improvement in impact strength when low amounts, from about 3 to about 20% by weight, preferably 5 to 15% by weight, of linear low density polyethylene and from about 5 to 50% by weight fibrous glass are incorporated therein. The impact strength of the compositions of this invention are much improved over thermoplastic polyester compositions having incorporated therein high pressure low density polyethylene or glass or both. Although the exact mechanism by which this unexpected improvement in impact strength arises is unknown, those skilled in the art will recognize synergism between the linear low density polyethylene, at the levels used, and the glass fibers accounting for the resultant improvement.